You might want to search/look for monitors from workstations. They tended to have larger screens than PCs, and the later ones will do multisync (the earlier ones are fixed sync, but if you can control your video clocks, that might work for you too).
For instance Sun had at least a 17" and 21" multisync. The 19" Sun monitors I'm familiar with are fixed sync. Their 20" monitor that had a infrared remote I think is fixed or multiple sync (I don't think it is multisync). SGI likely had some too. You'd need to see if any of those are available (Craigslist/etc), and google to determine if they are multisync. Depending on the monitor you might need an adapter if they don't have the standard PC VGA port (Sun used 13W3 connector, though most if not all of the Sun monitors that did are probably not multisync ?).
I'm definitely seeing less of them around. Though I have 2 21" Sun monitors within 4 feet of me as I type this

(but not using them at the moment

).
My basement probably has around 10+ computer monitors of various sizes, mostly 17".
They are out there, they just aren't everywhere anymore.
Happy hunting.